When sounds collected by microphone are directly heard through a sound reproducing apparatus that utilizes an earphone and a headphone, there are problems such that a large amplitude signal and noises to be heard are too loud, an articulation is low in the case of a reproduction of words, and, a cooped-up feeling and a feeling of oppression are accompanied. Hence, for the problem associated with a large amplitude, the inventor of the present invention has proposed an amplitude limiting apparatus and an amplitude suppressing apparatus which have little reduction of the reproduced sound quality (see Patent Documents 1 to 3). Since distortion signal components that make an auditory sense unpleasant (the major cause in the distortion of reproduced sounds that makes the auditory sense unpleasant is a frequency component of equal to or higher than substantially 4 KHz) are generated together with an operation, those apparatuses include a function that eliminates or reduces such distortion signal components.
According to such a scheme, however, there is a technical limitation for a performance which eliminates undesired signal components while leaving necessary signal components, and there is a difficulty for surely maintaining an excellent frequency characteristic at a high frequency band. In addition, effects for technical problems, such as noises, articulation, cooped-up feeling and feeling of oppression, are still insufficient. Still further, those technologies have a non-practical aspect from the standpoint of an apparatus that does not need a high frequency signal.
Conventionally, the reasons for noises, cooped-up feeling, feeling of oppression, and low articulation of words, etc., in the case of sound reproduction by earphone and headphone have been unknown. Hence, there has been no direct solution, and this is an important technical problem in hearing aids, recorders, and the like (some hearing aid products have a function of eliminating noises, but the foregoing problems are not addressed by this elimination function).
The inventor of the present invention found the followings through the development procedures of hearing aids. That is, the sensitivity of an ear changes in accordance with environmental sounds with a high frequency (a high-frequency component generally thought as environmental noises). When such environmental sounds are large or intensive, the sensitivity of an ear decreases, and when such sounds are not intensive, the sensitivity of an ear increases. Hence, when sounds that have only such environmental sounds eliminated or suppressed are applied to an ear, low-tone frequency sounds are awkwardly heard as being too loud, causing the cooped-up feeling and the feeling of oppression.
In addition, high-frequency environmental sounds remarkably affect spaciousness and realistic sensation. It has been known that sounds from a bone-conduction headphone are easily hearable when an earplug is simultaneously utilized. This is because the earplug blocks off high-frequency environmental sounds and thus the sensitivity of an ear increases. Still further, earphones and headphones that have a function of causing surrounding sounds to be difficult to hear (in general, called noise-cancel earphones) are becoming popular. When such a function is activated, a user may get a cooped-up feeling. This is because that environmental sounds, in particular, high-frequency environmental sounds become not hearable.
When words collected by microphone are reproduced with an excellent frequency characteristic using an earphone and a headphone, and a sound volume is changed, an intelligibility of the words also changes. In addition, when the sound volume changes from loud to mute, there is a point at which environmental sounds are suddenly felt as being soft although the sound-volume feeling of words substantially remains the same. Around this point, the intelligibility becomes the maximum. It is though that this phenomenon has a close relationship with an ear that has an excellent ability of making out words under noises.
Sound signals in television and radio broadcasting, or from commercially available music software, etc., are subjected to an amplitude limitation. The major purpose is to avoid a technical problem due to an insufficient performance of sound apparatuses. This amplitude limitation brings a preferable effect for reproduction by earphone and headphone in low-tone frequencies. However, as for high-tone frequencies, the amplitude limitation effective for reproduction by earphone and headphone is not performed. Hence, when such signals are to be heard using an earphone and a headphone that have a flat frequency characteristic, high-tone sounds are heard as being too intensive. It is thought that an ear of a well-conditioned person performs a suitable amplitude limitation on incoming sounds, but the mechanism and the reason why this amplitude limitation ability is deactivated when an earphone and a headphone are utilized are still unclear.
In order to reproduce sounds by earphone and headphone to be hearable with a natural sound quality, it is important to perform a suitable amplitude limitation to entire frequency band of sounds, and to maintain a uniform gain characteristic throughout all frequencies relative to signals within an amplitude limitation value. According to conventional systems that utilize an earphone and a headphone, however, since the gain characteristic is not flat, high-frequency sounds are to be heard at a certain sound volume. That is, normal earphones and headphones are manufactured to have a low high-tone sensitivity. This is convenient for the easiness of the manufacturing of earphones, and high-tone sounds are also heard so as not to be too intensive. However, the high-tone frequency gain of such reproduced sound is insufficient, and thus environmental sounds are not hearable appropriately.
Consequently, reproduced sounds by earphone and headphone are likely to bring an unpleasant feeling. Problems such that “noises are unpleasant and words from a distant sound source are not hearable when a hearing aid is used”, and “when recorded sounds are reproduced, words from a distant sound source are not hearable” are due to the low high-frequency gain of an earphone and that of a headphone. That is, this technical problem does not occur when a suitable amplitude limitation for reproduction by earphone and headphone, and, a reproduction with a uniform gain characteristic throughout all frequencies are performed.
In the case of sound reproduction by speaker, words from a distant sound source are hard to hear like the case of earphones, although there should be no problem in the case of sound reproduction by speaker without an amplitude limitation. This may be because of a remarkable difference between the reproduced sound field by speaker radiation and a sound field at the time of recording. Conversely, in the case of reproduction by earphone and headphone, sound waves similar to those at the time of recording are applicable to an ear, and thus the sound quality is excellent in comparison with a sound reproduction by speaker. However, there are many technical problems due to the above-explained reasons.